Iridium is a refractory platinum-group metal. The combination of such properties as high melting point and the highest among metals unreactiveness make this metal an indispensable structural materials for operation under conditions of elevated temperatures (up to 2200° C.) and aggressive environments. This is the only metal from which special application articles are manufactured, such as:
crucibles for growing large oxide monocrystals used in microelectronics and laser technology;
rolled stock for the manufacture of small-size nuclear reactor pressure vessels and containers for plutonium dioxide-based thermoelectric generators for interplanetary space flights;
disks for the manufacture of radioisotope sources for use in non-destructive testing instruments and in oncological diseases treatment;
wire for the manufacture of thermocouples for not less than 2200° C. measured temperatures range;
electrodes for motor vehicle spark plugs ensuring over 250 thousand kilometers of vehicle run.
The worldwide demand for iridium articles in 2014 grew four times to reach 10.5 tons, with a 2.5-times increase in the price for iridium. Steady industrial demand for iridium products is a factor preserving stability of prices at the present time as well.
The major problem in the manufacture of products from iridium is high sensitivity of this metal processing and service characteristics to presence of most insignificant quantities of impurity elements, which appear to be the principal cause of brittle failure of iridium. According to the traditional technology of manufacture of iridium articles, the blanks used in the production of iridium rolled stock present iridium monocrystals with impurities content not exceeding 0.003% obtained in the process of preliminary multi-stage energy-consuming fire refining with the use of electron-beam remelting (EBR) under vacuum. The industrial technology of production of this metal is available to a limited number of manufacturers, among them such companies as: Johnson Matthey PLC (UK), W.C. Heraeus GmbH (Germany), the Oak Ridge National Laboratory (USA), Engelhard-CLAL Corporation (USA), OAO Ekaterinburg Non-Ferrous Metals Treatment Plant and ZAO URALINTEKH (Russia).
High-purity iridium metal nanopowder (MINP) is designed for creation of new structural materials with a set of targeted physical-chemical and application properties, offering an opportunity for development of principally new efficient processes for the manufacture of articles with improved service characteristics.
MINP was generated as a result of an original combination of hydrometallurgical, electrochemical and fire refining processes; it presents black particles of 99.990% chemically pure metal 20-70 nanometers in size, with surface area within 3-7 m2/g and powder bulk density of 0.4-0.9 g/cm2. The metal is thermally stable at up to 200° C. in air, and non-pyrophoric.
As compared with the industrial iridium powders of home and foreign manufacture, the proposed MINP exceeds them by an order of magnitude in chemical purity, 5 to 10 times in powder bulk density characterizing unit mass consumption per unit volume, and 200 to 400 times in surface area characterizing catalytic activity.
Processes under development in the USA include deposition of a coat of iridium on articles operating under rigorous service conditions, on rocket nozzles in particular. A refractory corrosion-resistant coating of the working surfaces of a combustion chamber and a rocket-thrust-chamber nozzle will allow temperature rise from 1300° C. (current engines) to 2000-2100° C., resulting in increased fuel efficiency and useful load (see www.fazwest.com).
To this aim, the chemical vapor deposition method (CVD) with the use of complex organic iridium compounds was earlier selected; however the process of production of these metalorganic compounds is expensive since it requires special synthesis conditions, and the CVD process proper with the use of these compounds is found hard to localize in the target zone. For this reason, 70-80% of iridium is assimilated by chamber walls, and only 20-30% being deposited on the target surface.
There is a known technology of manufacture of iridium crucibles (see Timofeev N. I., Yeramakov A. V., Dmitriev V. A., Panfilov P. E. Osnovy metallurgii i tekhnologii proizvodstva izdelii iz iridiya (Basic metallurgy and process for production of articles from iridium). Ekaterinburg: UB RAS, 1996.—prototype) comprising the following operations:
production of metal compact by pressing initial powder, GOST 12338-81;
induction oxidizing melting in periclase crucible;
autoclaving;
ingot swaging;
electron-beam vacuum melting in horizontal mold;
electron-beam non-crucible zone melting, monocrystal growing;
swaging (2000-1100° C.), production of sheet bar;
boiling in chloronitric acid;
rolling (1000-800° C.), production of plate;
boiling in chloronitric acid;
manufacture of crucible shell ring (600-800° C.);
boiling in hydrochloric acid;
shell ring welding and its welding to bottom;
crucible annealing at 1100° C.;
polishing.
Metal yield in a finished article is 60-65% with up to 1.3% irrecoverable loss.
Drawbacks: great number of operations, low finished product yield and high production cost.